I know several questions were asked about the difference between "going to" and "will".
Based on several answers (see, for instance, here, here and here), I understood that "will" is more spontaneous and "going to" is used with more planned actions.

So, it seems that everything is pretty fine. However, in this question, Kosmonaut has an answer in which he states:
"Let's say that tomorrow you will walk your dog from 7 - 8 AM".

On the one hand, you probably planned to walk your dog long before and thus I should use "Let's say that tomorrow you're going to walk your dog from 7 - 8 am".

On the other hand, since I'm saying "let's say...", I'm deciding right now (thus, unplanned) that you will walk your dog. So, even though in this hypothetical situation you made a plan, I'm in a more spontaneous mood deciding right now that that's what you will do tomorrow, and, thus, I should use "will".

Re: "I understood that 'will' is more spontaneous and 'going to' is used with more planned actions": I don't think that's right. For example, I have no problem with, "Uh-oh, I took the wrong purse . . . someone is going to have the shock of the life when they get home and open theirs!"
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ruakhOct 24 '12 at 1:47

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+1 An excellent question. All of the explanations are wrong because the premise bears no relation to the facts of usage, at least in the US. The only difference between *Let's say you will walk/you'll walk/you are going to walk/you're going to walk/you are walking/you're walking/you walk the dog tomorrow" is that each provides different patterns which you may exploit to express semantic nuance prosodically, through rhythm, pace, stress and tone. You, or your circle, or even your region may have your own preferred uses, but the language does not.
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StoneyBOct 24 '12 at 2:20

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From what I understand, the distinction between will for spontaneous (natural, accidental) and going to for intended (planned, prepared, primed) action is a rule very frequently neglected. People regularly use the two interchangeably, and I don't think you'll find many who consider that a serious mistake.
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SF.Oct 24 '12 at 8:30

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@SF: I'd be surprised if you find many (aside from ESL teachers) who consider that a mistake at all. I think this rule is mainly taught to ESL students, and the real usage among native speakers is much less clear-cut. There are indeed some sentences where it's clear that one or the other is usually used, but in the majority of sentences either works.
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Peter Shor Oct 24 '12 at 13:20

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Can one neglect a rule which one did not even know existed?
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horatioOct 24 '12 at 17:46

4 Answers
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In most contexts, I am going to [verb] and I will [verb] are interchangeable. Sometimes the former may place more emphasis on the fact of your current intention/expectation, where the latter emphasises the future action.

There are some contexts where the difference is clear, and this may have some bearing on why OP thinks there's a planned/spontaneous distinction (usually there isn't). Say you're round a friend's house watching a football match on TV, and at half-time the beer runs out...

You: "I'll nip down the shop and get some more beer."

Friend: "It's raining - I'll find my umbrella for you."

If you reply "Don't bother - I'm going to use my car", the implication is you had already decided you were going to use the car before you first said you'd get the beer. But if you say "Don't bother - I will use my car", this implies you just made that decision in response to what your friend said.

@Peter Shor mentions another context ("It's going to/It will bite!") where native speakers often make a distinction. In that case, going to usually signifies immediate danger (it's just about to bite), but will can just mean it's bitten others before, and will/may bite you soon if you're not careful (effectively, the same as "Careful! It bites!" where present tense indicates "habitual" action).

Kosmonaut's "Let's say that tomorrow you will walk your dog" isn't really relevant to the current issue. It could just as well have been "...tomorrow you are going to walk..." or even just "...tomorrow you walk...". They all mean the same, and in that context there's no real reason to prefer one over another.

+1 for this explanation. Thanks. But "I'm going to be a doctor" doesn't necessarily mean in the near future, when a little kid answers it, does it? I know the differences but I just cannot explain it to a foreigner (eg: an Arab).
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itsolsJun 26 '13 at 14:27

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@itsols: Per first line of the answer, in most contexts (including a kid saying "I'm going to/will be a doctor") the two forms are completely interchangeable and there is no difference in meaning. In certain contexts, the "present tense" auxiliary verb "It is going to bite" can convey greater "immediacy". So you'd more likely use that form to warn someone that it's just about to bite them right now, because the "future/volitional" auxiliary "It will bite" more easily carries the sense that it's inclined to bite, and might well do so at some point in the future.
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FumbleFingersJun 26 '13 at 16:01

Doing some research, I discovered that the spontaneous vs planned rule for "going to" and "will" is taught to ESL students, but not used otherwise. I did discover a few good rules of thumb that are decent guidelines and make sense to me (a native English speaker):

"Going to" is a kind of present tense, so use "going to" in situations where the
present is connected to the future.
Example:
I feel a drop - I bet it's going to rain.
I am going to walk to school because my bike has a flat tire.

Use "will" in writing and "going to" when speaking.

Use "will" when talking about the not-immediate future and "going to" for the immediate future.

Use "will" and "going to" interchangeably for making predictions.

If you are looking for a good explanation about when to use either form, I suggest you read this article, which explains how to teach the differences between "going to" and "will".

So like I've mentioned in the comment for FumbleFingers, "I'm going to be a doctor" doesn't necessarily mean in the near future or immediate... But your point seems generally applicable to most cases. +1 for that as well.
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itsolsJun 26 '13 at 14:30

Any answer given on EL&U, given the constraints of time and space, will leave out some of the truth. That perhaps explains the limited nature of the earlier answers to this question to which the OP refers. In any case, non-native speakers usually want simple answers and these will suffice up to intermediate level.

This particular example introduces additional complications, because the speaker is not talking about the speaker’s own future acts, but about someone else’s. That makes the spontaneous/planned distinction inappropriate. In this case, going to seems the most natural expression, and it is indeed ‘often used as a general verb form for the future, especially in spoken English’ (‘An A-Z of English Grammar and Usage’).

English has no set of verbal inflections to express the future. It uses instead auxiliary verbs or, in some cases, the present tense or the present progressive construction. Although going to can be used in many cases, it can’t be used in all, and it certainly isn't invariably interchangeable with will. Native speakers will know intuitively when to say, for example, ‘Next year we’re going to have a holiday in Greece’ (not will) and when to say ‘Right, I’ll see you outside in half an hour’ (not going to). There is a clear difference in usage, reflecting the speaker’s intention in each case. Non-native speakers can only expect to distinguish such differences after considerable exposure to the language. The kind of quick fix found in the spontaneous/planned distinction will serve up to a point, but not beyond.

Ok Barrie, consider this: It's 09:00 and I am in a meeting with a friend, the meeting ends at 09:30 after the meeting I'll meet her at 10:00 on the the street because she has to run some quick errands. She has calculated if we will have time and I have two answers. 1st "It's ok, because I will meet you at the plaza in half an hour." the 2nd " It's ok because I am going to meet you at the plaza in half an hour.". Isn't it really implied in the circumstance rather than on the preferability of one over the other?
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SurvMachOct 16 '13 at 12:10

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If you have a new question, I suggest you post it separately.
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Barrie EnglandOct 16 '13 at 12:16

Do you think it's worthy of a new question? if so, how do I link this answer to the body of my question?
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SurvMachOct 16 '13 at 12:19

I'm not sure planned/unplanned quite describes it. Rather think of it as intention vs decision.

'Let's say that tomorrow you will walk your dog from 7 to 8 AM'

Are you expressing an intention, or a decision? An intention. So, use 'will' if you’re expressing an intention or making the decision as you speak; use 'going to' if you have already made the decision.

'I am definitely going to school tomorrow, because I have a test.'

But this doesn't cover every example, neither does it work all the time. So there is another way to look at it. This article put it this way:

"Going to" is used when you want to talk about a future situation that
is already connected to the present, e.g. because there’s present
evidence, or because a plan is already in motion.

Such as 'It’s going to rain – I just felt a drop.' or Ruakh's example '...someone is going to have the shock of their life.'. Both of these are rooted to something happening now.

Compare that with 'It will rain if you wash your car'. This phrase is not connected with the present, so we use 'will'.

And certainly there are cases where 'will' and 'going to' can be used interchangeably.

But in "going to school", "going to" is not the auxiliary verb expressing the future, but the main verb. It should be "going to go to school tomorrow" or "going to attend school tomorrow".
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Peter Shor Oct 24 '12 at 3:32